Physics, 500+ cars, great online racing. There's unique and exceptional. I don't see much else in GT that way, but to each their own. Also, GT1/2 did fine without maintenance, and they probably felt more unique than anything in their own time.
You probably do not remember, but "ownership/maintenance" RPG-esque philosophy is here since the first game.
GT1 had "Car Wash", first (and last, no other driving game never took such step in designing the approach to career mode) introduction of actual care for your virtually-owned vehicle in the series
GT2 had Car Wash too. In both (and in later games, up to GT5 today), Car Wash was eliminating the build up of "dust" on your car (visually represented by "matting" since GT1) which is downgrading aerodynamical-performance of the vehicle up to 7-10%. Washing the car brings back aerodynamic-performance to 100%.
GT3 introduced Oil Change and real-time monitoring of mileage of particular car. By using a particular vehicle, motor wears-out and oil "darkens". Initial changing of oil (on new vehicles in GT3, Used Cars of GT1 were re-introduced in series in GT4) boosts HP of the powerblock up to 8% and degrades it to factory-spec through initial 2000 km. After some time HP decreases with usage (oil gets darker) and in order to have desired spec you need to often change the oil on used vehicle. After some period your HP would decrease to level of -12/-15% of "factory-spec" (engine would wear out) and you couldn't bring it back.
GT4 expanded the philosophy even further with introduction of Chassis Rigidity Refresh where overall rigidity of the chassis was influenced by overall mileage (as powerblock and oil) and your actual way of driving. If you'd crash a lot car would loose rigidity much faster, prompting the Chassis Rigidity Refresh procedure. You could "delay/prolong" the procedure with installing additional roll-cage (Increase Chassis Rigidity procedure).
For Used Cars in GT4 you needed to change the oil and wash them in order to get both aerodinamyc and powerblock specifications to maxx, while cars with big-mileages asked for Chassis Rigidity Refresh in order to have their original handling-characteristics.
GT5 pushes overall "ownership/maintenance" philosophy even further, with combining all above with introduction of Engine Overhaul Process and constant real-time monitoring of overall usage of the vehicle.
Also notice that the engine of particular vehicle in GT5 will wear-out sooner/faster if you over-rev it, drive aggressively and shift in red-area.
All described may be unknown to you, but it is by far the most unique and original aspect of the Gran Turismo series and it makes it different than all other games in genre.
I can understand that need to invest effort and actual care for your vehicle may be too much for your personal taste, but it is the way GT games are designed from 1997.
Removing "ownership/maintenance" philosophy from GT series would make it "just another driving game".
I have strong hopes it will be the opposite and that upcoming games in series will actually make it more complex, demanding and expand it with possibilities of the new hardware.
Fact that no other racing-game developer NEVER even tried to introduce such features (besides basic mileage-counter without any repercussions to the vehicle-overall-state) speaks enough of how really special GT series is in last 13 years.